Reliable predictions indicate that there will be over 300 million cellular telephone customers by the year 2000. Within the United States, cellular service is offered by cellular service providers, by the regional Bell companies, and by the national long distance operators. The enhanced competition has driven the price of cellular service down to the point where it is affordable to a large segment of the population.
To maximize usage of the available bandwidth, a number of multiple access technologies have been implemented to allow more than one subscriber to communicate simultaneously with each base transceiver station (BTS) in a wireless system. These multiple access technologies include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA). These technologies assign each system subscriber to a specific traffic channel that transmits and receives subscriber voice/data signals via a selected time slot, a selected frequency, a selected unique code, or a combination thereof.
In order to further increase the number of subscribers that can be serviced in a single wireless network, frequency reuse is maximized by making individual cell sites smaller and using a greater number of cell sites to cover the same geographical area. Accordingly, the greater number of base transceiver stations increases infrastructure costs. To offset this increased cost, wireless service providers are eager to implement any innovations that may reduce equipment costs, maintenance/repair costs, and operating costs, or that may increase service quality/reliability, and the number of subscribers that the cellular system can service.
Electrical power is one of the more significant operating costs of a wireless system. Every base transceiver station has a transmitter for sending voice and data signals to mobile units (i.e., cell phones, portable computers equipped with cellular modems, and the like) and a receiver for receiving voice and data signals from the mobile units. The transmitter uses a comparatively large RF power amplifier to increase the strength of transmitted signals.
Wireless systems cannot tolerate large amounts of signal distortion and therefore require the use of RF amplifiers having good linearity characteristics across a wide range of operating conditions in order not to violate the IS 95 bandwidth requirements due to spectral spreading effects. Unfortunately, the DC-to-RF conversion efficiency for linear RF amplifiers is very low. CDMA amplifiers generally require about 8-10 dB of overhead input power ratio in order to maintain linearity in the RF waveforms.
The transmitter power amplifier consumes a constant and comparatively large amount of power, regardless of the relative strength of the output signal transmitted by the base transceiver station. For example, if the normal traffic load during the daytime requires the RF output power level to be approximately 10 watts, the DC prime power consumed by the transmitter power amplifier is approximately 80-100 watts (i.e., 8-10 dB higher). However, in the middle of the night, when the traffic load is very light, the RF output power level of the transmitter may be reduced in decrements down to, for example, about 1 watt, as power control is exercised over the RF output signal. However, the DC prime power consumed by the transmitter power amplifier will still be approximately 80-100 watts, since the operating bias points of the power amplifiers are fixed. In short, no allowance is made for reduced traffic loads.
There is therefore a need in the art for improved wireless networks that are less expensive to operate. In particular, there is a need for wireless networks that implement power control in the power amplifiers of the base station transmitters. Improved systems are needed that monitor the RF output signal level of a transmitter power amplifier and reduce the DC power level of the transmitter power amplifier according to the traffic load on the base station.